The invention relates generally to a method and system for wireless communications of packets. For successful communications of the packets, the received signal power must be sufficient to mask the noise. For a given link between a source node and a target node, the signal power received at the target node depends on the signal power transmitted at the source node, on the characteristics of the node antennas, and on the length and characteristics of the propagation path between the nodes. Thus, one technique for increasing signal power received at the target node and thereby raising the likelihood of successful packet communications is to increase source node transmitter power.
There are however disadvantages to increasing source node transmitter power. High transmitter power implies shorter battery life in mobile systems and higher energy costs in fixed systems. If the packet communications link shares spectrum with other such links or other services, high transmitter power leads to more interference to those links and services. High transmitter power also renders communications less secure by allowing for packet interception over a wider area. It is therefore desirable to transmit using only the minimum power necessary for reliable communication.
There have been several approaches to the problem of minimizing source node transmitter power while maintaining link reliability. One technique has been to compute the minimum transmitted power when designing the link and incorporate appropriate attenuation values in the specifications of transmission equipment constructed for the link. Link margin is added to the minimum transmitter power to allow for naturally occurring variations in link attenuation and performance variations in the equipment.
There are several problems with this approach. Link conditions may change over time. Antennas may be replaced. There may be both long-term and short-term variations in propagation conditions and equipment performance. One or both nodes of the link may be mobile making computation of a fixed minimum transmitter power impossible.
A second technique has been to make manual measurements at each node of the link to determine the minimum transmitter power. Transmitter attenuation can then be adjusted manually according to the measurement. A disadvantage of this approach is that any change in link conditions necessitates manual readjustment of the transmitter attenuation.
A more sophisticated technique is to use the target node receiver itself as a precision power measuring instrument. The receiver by tracking its own power measurements over time determines the minimum signal it requires. Knowing the link characteristics and the desired link margin, the receiver can then compute the desired transmitter power and convey this information to the other end of the link where the transmitter can adapt its power as specified.
This approach also has problems. Economical reliable receivers that also perform as accurate signal power measurement instruments are difficult to design. It is also difficult to economically manufacture and operate transmitters that can accurately and repeatably set a specified output power level. The requirement that the receiver and transmitter measure and adjust power in the same units further raises the cost of implementation.
A simplified variation on this approach was disclosed in U.S. Pat. No. 5,107,225. Therein is disclosed a receiver AGC (Automatic Gain Control) circuit where the gain control voltage is derived from an integrated quantitative indicator of received signal strength. The gain control voltage is used to adjust both the receiver gain and the power output of a co-located transmitter. No power adjustment data is sent to the transmitter on the opposite side of the link.
What is needed is a low-cost, reliable mechanism for minimizing transmitted power while maintaining reliable communications between a source node and a target node.